Method for controlling the starting of an AC induction motor

Abstract

A method is provided for controlling the starting of an AC induction motor with a soft starter. The windings are connected in a delta configuration and each winding has a corresponding thyristor switch connected in series therewith such that each winding and thyristor switch combination is connected between two corresponding terminals of the AC induction motor. Alpha control of the thyristor switches is utilized to start the AC induction motor and bring the AC induction motor to near full operating speed. As the AC induction motor nears full operating speed, gamma control of the thyristor switches is utilized to bring the AC induction motor to full operating speed. With the AC induction motor at full operating speed, bypass contactors, in parallel across corresponding thyristor switches, are closed and the AC induction motor operates in a conventional manner.

We claim:  
     
       1. A method for controlling a three phase, AC induction motor having at least three windings and six terminals wherein three of the terminals are connectable to an AC input source for providing voltage and current to the AC induction motor and wherein the windings are connected in a delta configuration and each winding has a corresponding thyristor switch connected in series therewith such that each winding and thyristor switch combination is connected between two corresponding terminals of the AC induction motor, the method comprising the steps of: 
       initially firing each thyristor switch at a predetermined, initial alpha firing angle after occurrence of zero volts supplied by the AC input source;  
       calculating a new alpha firing angle;  
       sequentially firing each thyristor switch at the new alpha firing angle after occurrence of zero volts supplied by the AC input source; and  
       switching to gamma control and sequentially firing each thyristor switch at an initial gamma firing angle after occurrence of zero supply current therethrough.  
     
     
       2. The method of  claim 1  further comprising the steps of: 
       providing the new alpha firing angle as the initial alpha firing angle; and  
       repeating for a predetermined time period the steps of calculating a new alpha firing angle and sequentially firing each thyristor switch.  
     
     
       3. The method of  claim 2  wherein the new alpha firing angle is calculated according to the expression: 
       
         
           α i =−s*t+k  
         
       
       wherein α i  is the new alpha firing angle; s and k are constants; and t is the elapsed time from the initial firing of each thyristor switch. 
     
     
       4. The method of  claim 3  further comprising the steps of: 
       upon completion of the predetermined time period, sequentially firing each thyristor switch at a user selected alpha firing angle after occurrence of zero volts supplied by the AC input source; and  
       repeating the step of sequentially firing each thyristor switch at a user selected alpha firing angle for a second predetermined time period.  
     
     
       5. The method of  claim 3  further comprising the steps of: 
       upon completion of the predetermined time period, sequentially firing each thyristor switch at a user selected alpha firing angle after occurrence of zero volts supplied by the AC input source; and  
       monitoring motor current in the AC induction motor and if the motor current is greater than a predetermined value, repeating the step of sequentially firing each thyristor switch at a user selected alpha firing angle.  
     
     
       6. The method of  claim 1  wherein prior to switching to gamma control, executing the additional steps of: 
       monitoring motor current in the AC induction motor and if the motor current is greater than a predetermined value, executing the additional steps of:  
       providing the new alpha firing angle as the initial alpha firing angle; and  
       repeating the steps of calculating a new alpha firing angle and sequentially firing each thyristor switch at the new alpha firing angle.  
     
     
       7. The method of  claim 6  wherein the new alpha firing angle is calculated according to the expression: 
       
         
           α i =α i−1 +k*(I−I lim )  
         
       
       wherein α i  is the new firing angle; α i−1  is the initial firing angle; k is a proportional gain; I is an integral of the motor current over a conduction time interval; and I lim  is a user selected pre-set current limit. 
     
     
       8. The method of  claim 7  comprising the additional steps of comparing the absolute value of the expression k*(I−I lim ) to a limit prior to calculating the new alpha firing angle and replacing the expression k*(I−I lim ) with the limit in the expression to calculate the new alpha firing angle if the absolute value of the expression k*(I−I lim ) is greater than the limit. 
     
     
       9. The method of  claim 8  wherein the step of switching to gamma control includes the additional steps of: 
       monitoring the operating speed of the AC induction motor and if the operating speed is less than the full speed of the AC induction motor, executing the additional steps of:  
       initializing the gamma firing angle;  
       calculating a new gamma firing angle;  
       sequentially firing each thyristor switch at the new gamma firing angle after occurrence of zero supply current therethrough;  
       providing the new gamma firing angle as the initial gamma firing angle; and  
       returning to the step of calculating a new gamma firing angle.  
     
     
       10. The method of  claim 9  wherein the new gamma firing angle is calculated according to the expression: 
       
         
           γ i =γ i−1 +k*(I−I lim )  
         
       
       wherein γ i  is the new gamma firing angle; γ i−1  is the initial gamma firing angle; k is a proportional gain; I is an integral of the motor current over a conduction time interval; and I lim  is a user selected pre-set current limit. 
     
     
       11. The method of  claim 10  comprising the additional steps of comparing the absolute value of the expression k*(I−I lim ) to a gamma limit prior to calculating the new gamma firing angle and replacing the expression k*(I−I lim ) with the gamma limit in the expression to calculate the new alpha firing angle if the absolute value of the expression k*(I−I lim ) is greater than the gamma limit. 
     
     
       12. The method of  claim 1  comprising the additional steps of: 
       providing bypass contactors in parallel across corresponding thryristor switches;  
       monitoring the operating speed of the AC induction motor; and  
       closing the bypass contactors in response to the AC induction motor operating at a predetermined operating speed.  
     
     
       13. A method for controlling a three phase, AC induction motor having at least three windings and six terminals wherein three of the terminals are connectable to an AC input source for providing voltage and current to the AC induction motor and wherein the windings are connected in a delta configuration and each winding has a corresponding thyristor switch connected in series therewith such that each winding and thyristor switch combination is connected between two corresponding terminals of the AC induction motor, the method comprising the steps of: 
       firing each thyristor switch at an alpha firing angle after occurrence of zero volts supplied by the AC input source;  
       sequentially reducing the alpha firing angle and repeatedly firing each thyristor switch at each reduced alpha firing angle after occurrence of zero volts supplied by the AC input source;  
       ceasing the firing of each thyristor switch at each reduced alpha firing angle after occurrence of zero volts supplied by the AC input source; and  
       switching to gamma control and sequentially firing each thyristor switch to bring the AC induction motor toward full operating speed at an initial gamma firing angle after occurrence of zero supply current therethrough.  
     
     
       14. The method of  claim 13  wherein prior to the step of ceasing the firing of each thyristor switch, executing the additional steps of: 
       sequentially firing each thyristor switch at a user selected alpha firing angle after occurrence of zero volts supplied by the AC input source; and  
       repeating the step of sequentially firing each thyristor switch at a user selected alpha firing angle for a predetermined time period.  
     
     
       15. The method of  claim 13  wherein prior to the step of ceasing the firing of each thyristor switch, executing the additional steps of: 
       sequentially firing each thyristor switch at a user selected alpha firing angle after occurrence of zero volts supplied by the AC input source; and  
       monitoring motor current in the AC induction motor and if the motor current is greater than a predetermined value, repeating the step of sequentially firing each thyristor switch at a user selected alpha firing angle.  
     
     
       16. The method of  claim 13  wherein the alpha firing angle is reduced according to the expression: 
       
         
           α i =α i−1 +k*(I−I lim )  
         
       
       wherein α i  is the reduced alpha firing angle; α i−1  is the alpha firing angle; k is a proportional gain; I is an integral of the motor current over a conduction time interval; and I lim  is a user selected pre-set current limit. 
     
     
       17. The method of  claim 16  comprising the additional steps of comparing the absolute value of the expression k*(I−I lim ) to a limit prior to calculating the new alpha firing angle and replacing the expression k*(I−I lim ) with the limit in the expression to calculate the new alpha firing angle if the absolute value of the expression k*(I−I lim ) is greater than the limit. 
     
     
       18. The method of  claim 17  wherein the step of switching to gamma control includes the additional steps of: 
       monitoring the operating speed of the AC induction motor and if the operating speed is less than the full speed of the AC induction motor, executing the additional steps of:  
       initializing the gamma firing angle;  
       calculating a new gamma firing angle;  
       sequentially firing each thyristor switch at the new gamma firing angle after occurrence of zero supply current therethrough;  
       providing the new gamma firing angle as the initial gamma firing angle; and  
       returning to the step of calculating a new gamma firing angle.  
     
     
       19. The method of  claim 18  wherein the new gamma firing angle is calculated according to the expression: 
       
         
           γ i =γ i−1 +k*(I−I lim )  
         
       
       wherein γ i  is the new gamma firing angle; γ i−1  is the initial gamma firing angle; k is a proportional gain; I is an integral of the motor current over a conduction time interval; and I lim  is a user selected pre-set current limit. 
     
     
       20. The method of  claim 19  comprising the additional steps of comparing the absolute value of the expression k*(I−I lim ) to a gamma limit prior to calculating the new gamma firing angle and replacing the expression k*(I−I lim ) with the gamma limit in the expression to calculate the new alpha firing angle if the absolute value of the expression k*(I−I lim ) is greater than the gamma limit. 
     
     
       21. The method of  claim 13  comprising the additional steps of: 
       providing bypass contactors in parallel across corresponding thryristor switches;  
       monitoring the operating speed of the AC induction motor; and  
       closing the bypass contactors in response to the AC induction motor operating at a predetermined operating speed.  
     
     
       22. A method for controlling a three phase, AC induction motor having at least three windings and six terminals wherein three of the terminals are connectable to an AC input source for providing voltage and current to the AC induction motor and wherein the windings are connected in a delta configuration and each winding has a corresponding thyristor switch connected in series therewith such that each winding and thyristor switch combination is connected between two corresponding terminals of the AC induction motor, the method comprising the steps of: 
       initially firing each thyristor switch at a predetermined, initial alpha firing angle after occurrence of zero volts supplied by the AC input source;  
       calculating a new alpha firing angle;  
       sequentially firing each thyristor switch at the new alpha firing angle after occurrence of zero volts supplied by the AC input source;  
       sequentially firing each thyristor switch at a user selected alpha firing angle after occurrence of zero volts supplied by the AC input source;  
       switching to gamma control and sequentially firing each thyristor switch at an initial gamma firing angle after occurrence of zero supply current therethrough;  
       providing bypass contactors in parallel across corresponding thryristor switches;  
       monitoring the operating speed of the AC induction motor; and  
       closing the bypass contactors in response to the AC induction motor operating at a predetermined operating speed.  
     
     
       23. The method of  claim 22  further comprising the steps of: 
       monitoring the operating speed of the AC induction motor and if the operating speed is less than the full speed of the AC induction motor, executing the additional steps of:  
       calculating a new gamma firing angle;  
       sequentially firing each thyristor switch at the new gamma firing angle after occurrence of zero supply current therethrough;  
       providing the new gamma firing angle as the initial gamma firing angle; and  
       returning to the step of calculating a new gamma firing angle.  
     
     
       24. The method of  claim 22  further comprising the steps of: 
       providing the new alpha firing angle as the initial alpha firing angle; and  
       repeating for a predetermined time period the steps of calculating a new alpha firing angle and sequentially firing each thyristor switch.  
     
     
       25. The method of  claim 22  wherein upon completion of the predetermined time period, executing the step of sequentially firing each thyristor switch at a user selected alpha firing angle after occurrence of zero volts supplied by the AC input source and repeating the step of sequentially firing each thyristor switch at a user selected alpha firing angle for a second predetermined time period. 
     
     
       26. The method of  claim 22  wherein upon completion of the predetermined time period, executing the step of sequentially firing each thyristor switch at a user selected alpha firing angle after occurrence of zero volts supplied by the AC input source and monitoring motor current in the AC induction motor and if the motor current is greater than a predetermined value, repeating the step of sequentially firing each thyristor switch at a user selected alpha firing angle.